FIG. 1 is a simplified depiction of a wind-driven generator 2. Wind, represented by reference numeral 3, causes a propeller 4 to rotate. The propeller 4 drives a generator 5, which generates electricity. The electricity generated by the generator 5 flows through a transmission line 6 to a load, such as an electrical power grid represented by reference numeral 7 or a consumer's home, a business or a small factory.
A problem with wind-driven electric power generation is that wind is unreliable and its speed is never constant. Excess propeller speed caused by high winds can be limited by a brake or by blade pitch, however, propeller speed cannot be downwardly controlled when wind velocity falls. When the wind speed falls, electric output power will fall since electric output power is directly related to propeller rotation speed. When the wind stops, output power will also stop. Fluctuating wind speed will therefore cause generator output to fluctuate.
A closely related problem is that the propeller 4 requires a certain amount of kinetic energy, i.e., rotational velocity, before it can even begin to generate usable amounts of output power, as FIG. 2 shows. Some energy must be imparted to the propeller before it can generate usable electric output power. When the wind slows or stops, latent kinetic energy in the rotating propeller and other rotation machinery connected to the propeller begins to dissipate through wind loss, bearing loss and electrical loading, if the generator is not disconnected from its electrical load. The lost kinetic energy must be restored by the wind before the generator can resume generating power. Maintaining propeller speed when the wind slows or stops might improve wind generator efficiency by shortening the time required to bring the generator on-line after the wind speed has recovered. A method and apparatus for simply and economically maintaining propeller speed, during intervals when the wind slows or has stopped, would be an advantage over the prior art.